This invention is directed generally to television signal processing and more specifically to a technique of encrypting television signals such that only designated subscribers can decode and display the television signal.
It is often desirable to transmit a video signal to a particular destination while maintaining a degree of security. i.e., ensuring that only the intended destination will be capable of receiving and displaying the video signal. Examples of applications which may employ encryption are teleconferences and subscription television. In subscription television systems, the video signal is broadcast and encryption is needed to prevent useful reception of the signal by customers who have not paid the monthly fee. Teleconferences may involve sensitive material such as financial plans, pending business decisions and personal medical data, and the teleconferences may be conducted using some combination of wire, microwave and communications satellites. All of these channels provide opportunities for unauthorized reception and display of the video information.
A number of techniques have been proposed for the encryption of video signals. U.S. Pat. No. 2,846,497 to Kennedy discloses a subscriber television system which employs a binary counter chain at both the transmitting and receiving locations. The binary counter chain is driven by a random signal which is related to the scanning rate of the television screen. Each time the transmitter counter is stepped, a drive tone is transmitted over the sound channel of the television station, and the transmitter counter is randomly reset in accordance with a reset signal which is also transmitted over the sound channel. The drive and reset signals received at the television receiver are employed to drive and reset the receiver binary counter. Both the transmitter and receiver are provided with television raster shifting means for shifting the television raster horizontally and vertically upon reaching various predetermined counts in the transmitter and receiver binary counters.
U.S. Pat. No. 2,875,269 to Ridenour discloses a video scrambling system which operates on the principal of providing an irregular sequence for horizontal scanning of successive fields in the television system. In the Ridenour system, the horizontal sync pulses are counted by a binary counter which establishes a different voltage pattern for each count. Each different voltage pattern is used to generate a different value of vertical deflection current for the video camera, and this value of vertical deflection current selected by the count can be programmed. At each subscriber's television receiver there is located the same apparatus for counting and decoding the horizontal sync pulses. Since the order of the vertical deflection currents being generated is the same, the picture presented on the subscriber receiver is intelligible. Should the program at either the receiver or the transmitter be different the picture presented on the receiver is scrambled.
U.S. Pat. No. 3,081,376 to Loughlin et al discloses a subscription television system wherein the normal or true synchronizing components are synchronized in amplitude to fall within the amplitude range normally set aside for video information, and these synchronizing components are replaced in the transmitted signal by other components having the appearance of the usual synchronizing information but representing false timing data. The coding signal is a sinusoidal component locked to the line frequency and phased to present a peak essentially at the middle of the line-trace intervals, serving as a pedestal for the video component. The false synchronizing signals are, in addition, interrupted from time to time. The decoder at the receiver includes a carrier-frequency modulator to which the received signal is applied in its scrambled form. Correcting signals are also applied to the modulator, and these correcting signals operate on the carrier in effect to remove the coding component.
U.S. Pat. No. 3,116,363 to Doundoulakis discloses a secure television system wherein an audio signal is synchronized with and transmitted with the video signal. The transmitter encoder depresses the horizontal sync signals with a key signal so as to distort the picture, the key signal comprising a substantially sinusoidal wave shape of a predetermined frequency and amplitude modulated on a key signal carrier. The receiver reproduces the audio signal in synchronism with the video and the receiver decoder reconstitutes the horizontal synchronizing signal from the received key signal to provide the required horizontal sync.
U.S. Pat. No. 4,045,814 to Hartung et al discloses a scrambling and unscrambling system for a subscription television wherein control signals are encoded into the vertical blanking intervals of the video signals. The control signals are decoded at the receiver and, if addressed to the particular subscriber's receiver, operate to enable the descrambler or to frequently vary its mode of operation thereby greatly increasing the security of the system.
While these various systems have proven highly useful, there are various disadvantages associated with each. The Kennedy, Ridenour and Hartung et al techniques do not remove the synchronization signals. The Loughlin et al technique suppresses the sync signals but adds a false sync which erases the beginning portion of the active video signal. Loughlin et al also adds a sinusoidal component to the video signal and then transmits a key signal over the audio channel, but the sync signal can be relatively easily restored.
Simple removal of the synchronization signals is not a sufficient security measure, since these signals are relatively easily restored. Modification of the video signal is not always satisfactory since large amounts of an interfering signal may produce an annoying display but much of the picture information remains. Further, complete removal of the interfering signal is often difficult. Other methods such as varying the gain (including signal inversion) present the same difficulty. Signal modification at a rate higher than the line rate is likely to produce visible transient waveforms after attempted removal, and variations at or below the line rate are likely to produce a picture impaired by low level streaks.
There is thus a need for a secure television signal encryption scheme which is easily implemented which requires relatively simple hardware, which does not tamper with the active portion of the video signal and which still provides a significant degree of communications security.